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Hearing & Sight
BIOL241
Ears & Eyes – Chap. 15
How do we see?
• Gross Anatomy
• Micro anatomy
• Physics
Ora serrata
Ciliary body
Ciliary zonule
(suspensory
ligament)
Cornea
Iris
Pupil
Anterior pole
Anterior
segment (contains
aqueous humor)
Lens
Scleral venous
sinus
Posterior segment
(contains vitreous humor)
(a) Diagrammatic view. The vitreous
humor is illustrated only in the
bottom part of the eyeball.
Sclera
Choroid
Retina
Macula lutea
Fovea centralis
Posterior pole
Optic nerve
Central artery
and vein of
the retina
Optic disc
(blind spot)
Figure 15.4a
Layers, again!!
• Outermost layers:
• dense avascular connective tissue
•
•
•
•
•
Sclera
Uvea
Cornea
Humors
more
Sclera
– Opaque posterior region
– Protects and shapes eyeball
– Anchors extrinsic eye muscles
Cornea:
– Transparent anterior 1/6 of fibrous layer
– Bends light as it enters the eye
– Sodium pumps of the corneal endothelium on
the inner face help maintain the clarity of the
cornea
– Numerous pain receptors contribute to
blinking and tearing reflexes
Uvea (Vascularized)
• Middle pigmented layer
• Three regions: choroid, ciliary body, and
iris
1.Choroid region
• Posterior portion of the uvea
• Supplies blood to all layers of the eyeball
• Brown pigment absorbs light to prevent visual
confusion
Uvea (Vascularized)
2
Ciliary body
– Ring of tissue surrounding the lens
– Smooth muscle bundles (ciliary
muscles) control lens shape
– Capillaries of ciliary processes secrete
fluid
– Ciliary zonule (suspensory ligament)
holds lens in position
Sympathetic activation
Nearly parallel rays
from distant object
Lens
Ciliary zonule
Ciliary muscle
Inverted
image
(a) Lens is flattened for distant vision. Sympathetic
input relaxes the ciliary muscle, tightening the ciliary
zonule, and flattening the lens.
Figure 15.13a
Parasympathetic activation
Divergent rays
from close object
Inverted
image
(b) Lens bulges for close vision. Parasympathetic
input contracts the ciliary muscle, loosening the
ciliary zonule, allowing the lens to bulge.
Figure 15.13b
Problems of Refraction
• Myopia (nearsightedness)—focal point is in front of
the retina, e.g. in a longer than normal eyeball
– Corrected with a concave lens
• Hyperopia (farsightedness)—focal point is behind
the retina, e.g. in a shorter than normal eyeball
– Corrected with a convex lens
• Astigmatism—caused by unequal curvatures in
different parts of the cornea or lens
– Corrected with cylindrically ground lenses, corneal
implants, or laser procedures
Emmetropic eye (normal)
Focal
plane
Focal point is on retina.
Figure 15.14 (1 of 3)
Myopic eye (nearsighted)
Eyeball
too long
Uncorrected
Focal point is in front of retina.
Corrected
Concave lens moves focal
point further back.
Figure 15.14 (2 of 3)
Hyperopic eye (farsighted)
Eyeball
too short
Uncorrected
Focal point is behind retina.
Corrected
Convex lens moves focal
point forward.
Figure 15.14 (3 of 3)
Functional Anatomy of
Photoreceptors
• Rods and cones
– Outer segment of each contains visual
pigments (photopigments)—molecules that
change shape as they absorb light
– Inner segment of each joins the cell body
Process of
bipolar cell
Synaptic terminals
Rod cell body
Rod cell body
Cone cell body
Nuclei
Outer fiber
Mitochondria
The outer segments
of rods and cones
are embedded in the
pigmented layer of
the retina.
Pigmented layer
Outer segment
Inner
segment
Inner fibers
Connecting
cilia
Apical microvillus
Melanin
granules
Discs containing
visual pigments
Discs being
phagocytized
Pigment cell nucleus
Basal lamina (border
with choroid)
Figure 15.15a
Rods
• Functional characteristics
– Very sensitive to dim light
– Best suited for night vision and peripheral
vision
– Perceived input is in gray tones only
– Pathways converge, resulting in fuzzy and
indistinct images
– “Night vision” (1/2 hour to establish)
Cones
• Functional characteristics
– Need bright light for activation (have low
sensitivity)
– Have one of three pigments that furnish a
vividly colored view
– Non-converging pathways result in detailed,
high-resolution vision
Central
artery
and vein
emerging
from the
optic disc
Macula
lutea
Optic disc
Retina
Figure 15.7
Chemistry of Visual Pigments
• Retinal
– Light-absorbing molecule that combines with one of
four proteins (opsin) to form visual pigments
– Synthesized from vitamin A
– Two isomers: 11-cis-retinal (bent form) and all-transretinal (straight form)
• Conversion of 11-cis-retinal to all-trans-retinal
initiates a chain of reactions leading to transmission
of electrical impulses in the optic nerve
β-carotene
• Why β-carotene? (Where have we seen it before?)
Lens
• Biconvex, transparent, flexible, elastic, and avascular
• Allows precise focusing of light on the retina
• Cells of lens epithelium differentiate into lens fibers that
form the bulk of the lens
• Lens fibers—cells filled with the transparent protein
crystallin
• Lens becomes denser, more convex, and less elastic with
age
• Cataracts (clouding of lens) occur as a consequence of
aging, diabetes mellitus, heavy smoking, and frequent
exposure to intense sunlight
Figure 15.9
Eyebrow
Eyelid
Eyelashes
Site where
conjunctiva
merges with
cornea
Palpebral
fissure
Lateral
commissure
Iris
Eyelid
Sclera
Lacrimal
(covered by caruncle
conjunctiva)
(a) Surface anatomy of the right eye
Pupil
Medial
commissure
Figure 15.1a
Levator palpebrae
superioris muscle
Orbicularis oculi muscle
Eyebrow
Tarsal plate
Palpebral conjunctiva
Tarsal glands
Cornea
Palpebral fissure
Eyelashes
Bulbar conjunctiva
Conjunctival sac
Orbicularis oculi muscle
(b) Lateral view; some structures shown in sagittal section
Figure 15.1b
Lacrimal sac
Lacrimal gland
Excretory ducts
of lacrimal glands
Lacrimal punctum
Lacrimal canaliculus
Nasolacrimal duct
Inferior meatus
of nasal cavity
Nostril
Figure 15.2
How do we hear?
•
•
•
•
Gross Anatomy
Micro anatomy
Physics
What is sound?
Air pressure
Wavelength
Area of
high pressure
(compressed
molecules)
Area of
low pressure
(rarefaction)
Crest
Trough
Distance
Amplitude
A struck tuning fork alternately compresses
and rarefies the air molecules around it,
creating alternate zones of high and
low pressure.
A pressure disturbance (alternating areas of high
and low pressure) produced by a vibrating object: A
sound wave
Moves outward in all directions
Is illustrated as an S-shaped curve or sine wave
(b) Sound waves
radiate outward
in all directions.
Figure 15.29
Properties of Sound
• Pitch
– Perception of different frequencies
– Normal range is from 20–20,000 Hz
•
Bass - Treble
(Hertz =?)
– The higher the frequency, the higher the pitch
• Loudness
– Subjective interpretation of sound intensity
– Normal range is 0–120 decibels (dB)
Auditory ossicles
Malleus Incus Stapes
Cochlear nerve
Scala vestibuli
Oval
window Helicotrema
2
3
Scala tympani
Cochlear duct
Basilar
membrane
1
Tympanic
Round
membrane
window
(a) Route of sound waves through the ear
1 Sound waves vibrate
the tympanic membrane.
2 Auditory ossicles vibrate.
Pressure is amplified.
3 Pressure waves created by
the stapes pushing on the oval
window move through fluid in
the scala vestibuli.
Sounds with frequencies
below hearing travel through
the helicotrema and do not
excite hair cells.
Sounds in the hearing range
go through the cochlear duct,
vibrating the basilar membrane
and deflecting hairs on inner
hair cells.
Figure 15.31a
Superior vestibular ganglion
Inferior vestibular ganglion
Temporal
bone
Semicircular
ducts in
semicircular
canals
Facial nerve
Vestibular
nerve
Anterior
Posterior
Lateral
Cochlear
nerve
Maculae
Cristae ampullares
in the membranous
ampullae
Spiral organ
(of Corti)
Cochlear
duct
in cochlea
Utricle in
vestibule
Saccule in
vestibule
Stapes in
oval window
Round
window
Figure 15.27
Medial geniculate
nucleus of thalamus
Primary auditory
cortex in temporal lobe
Inferior colliculus
Lateral lemniscus
Superior olivary nucleus
(pons-medulla junction)
Midbrain
Cochlear nuclei
Vibrations
Medulla
Vestibulocochlear nerve
Vibrations
Spiral ganglion of cochlear nerve
Bipolar cell
Spiral organ (of Corti)
Figure 15.33
Where is the Vestibulocochlear nerve?
Homeostatic Imbalances of
Hearing
• Conduction deafness
– Blocked sound conduction to the fluids of the
internal ear
• Can result from impacted earwax, perforated
eardrum, or otosclerosis of the ossicles
• Sensorineural deafness
– Damage to the neural structures at any
point from the cochlear hair cells to the
auditory cortical cells
Homeostatic Imbalances of
Hearing
• Tinnitus: ringing or clicking sound in the
ears in the absence of auditory stimuli
– Due to cochlear nerve degeneration,
inflammation of middle or internal ears, side
effects of aspirin
• Meniere’s syndrome: labyrinth disorder
that affects the cochlea and the
semicircular canals
– Causes vertigo, nausea, and vomiting
Kinocilium
Stereocilia
Otoliths
Otolithic
membrane
Hair bundle
Macula of
utricle
Macula of
saccule
Hair cells
Supporting
cells
Vestibular
nerve fibers
Figure 15.34
Cupula
Crista
ampullaris
Endolymph
Hair bundle (kinocilium
plus stereocilia)
Hair cell
Crista
Membranous
ampullaris
labyrinth
Fibers of vestibular nerve
(a) Anatomy of a crista ampullaris in a
semicircular canal
Supporting
cell
Cupula
(b) Scanning electron
micrograph of a
crista ampullaris
(200x)
Figure 15.36a–b
Section of
ampulla,
filled with
endolymph
Cupula
Fibers of
vestibular
nerve
At rest, the cupula stands
upright.
(c) Movement of the
cupula during
rotational
acceleration
and deceleration
Flow of endolymph
During rotational acceleration,
endolymph moves inside the
semicircular canals in the
direction opposite the rotation
(it lags behind due to inertia).
Endolymph flow bends the
cupula and excites the hair
cells.
As rotational movement
slows, endolymph keeps
moving in the direction
of the rotation, bending
the cupula in the
opposite direction from
acceleration and
inhibiting the hair cells.
Figure 15.36c